Output stages for low-frequency amplifiers comprising two loudspeakers



Aug. 5, 1958 J. KUMP OUTPUT STAGES FOR LOW-FREQUENCY AMPLIFIERSCOMPRISING TWO LOUDSPEAKERS Filed April 9, 1956 INVENTOR JOHANN KUMPAGENT United States Patent OUTPUT STAGES FOR LOW-FREQUENllY AMPLI- FIERSCOMPRISING TWO 'LOUDSPEAK ERS Inhann-Kump, Vienna, Austria,; assignor,by mesne as.-

signments, to. N orth. American Philips. Company, Inc., New York, N;Y.,. a corporationof Delaware Application April 9, 1956, Serial'No.577,113.

Claims priority, application. Austria May: 5, 1955 5 Claims. (Cl.179-1)" The present invention. relates: toan amplifier output stagecircuit arrangement. More particularly, the invention relates to outputstages of' low-frequency amplifiers comprising two loudspeakers, whichare preferably intended for the reproductionof low fequenciesand highfrequencies, respectively, of the acoustic spectrum with the use ofoneor more screen-grid tubes as output tubes.

The objectv of'the invention is. in such output stages to obtain aconsiderable decrease in distortion in the highfrequency range. It iscommonknowledge that distor tion inthe high-frequency range is.subjectively very troublesome to the ear. Consequently, inthe.reproduction of an. acoustic. frequency-band, which. must comprise thewhole acoustic. region. up. to about. kiljocycl'es. per second, therequirements in regard to freedom of distortion. must be much higherthan in. the reproduction of an. acousticv frequency-band which is.limited in the direction of high frequencies.

On. account. of their high output, the present types. of output. stages.of low-frequency amplifiers. substantially utilize screen-grid tubes andmore particularly pentodes. As appears from the characteristic curves.of such tubes, the new tones experienced therein. as distortion. substamtially comprise the third harmonicsand combination tones of higher orderand hence high tones. Insofar as-loudspeakers. are used for the acousticreproduction, which cannot or substantially cannot reproduce. thehigh-frequency range, the loudspeaker suppresses part of thisdistortion. If use is made of aloudspeaker reproducing the frequenciesup to 15 kilocycles per second,. or if two or more. loudspeakers. areused, which are'each. preferably intendedfor the reproduction of thelow-frequency ranges (low-tone loudspeaker) and the high-frequencyrangesv (high-tone. loudspeaker), respectively, the. high. frequencies.produced in the amplifier are. no longer suppressed and thusbecomemanifest. with maximum effect.

The behaviour of triodes ismore favorable. than that of screen-grid.tubes (pentodes) in. regard-v to distortion, but. their low. outputconstitutes a. great disadvantage.

A circuit. is already known, which has beenadopted literature as anultra-linear circuit (of. British. Patent No. 496,883), in Whichit isendeavoured tozcombine the advantages of triodes. and screen. gridtubes. In this,circuit, the screen-grid ofthe output tube, instead; ofbeing applied to cathode potential for alternating voltage, as isusuallythe case, has applied to it part of the alternating anode voltage. Thismay be accomplished, for example, by connecting; the screen-griddirectly to a tapping oint on the primary winding of' the outputtransformer ineluded in. the anode. circuit of the output tube, so thatthe screen grid at the same time also receives the Tequired -nected to ahigh-tone loudspeaker 16.

2. direct voltage Duetothis circuit, whichinfact means, the introductionof a negative feedback. coupling, the. tube. acquiresproperties located:between those of. a.triode andof a. tube in screen grid connection.Thedistortionis. reduced at the expense of the outputv and evenlyfor anyfrequency.

The invention is basedonthe.consideration. that a decreased output. maybe taken into the. bargain for/high. frequencies, if the outputremains-unchangedin the low. frequency and. intermediate-frequencyrange, since thestrength of the subjective impression. of sound isdetermined substantially by the reproduction of this range. Ifthis rangeis reproduced by a. loudspeaker which is insensitive to high:frequencies and hence, by. a. low-tone loudspeaker, a higher distortionfactor is permissible: for this.range, since, as: mentioned. before, thedistortion which. consists of. very high tones is not audible. However,the highefrequency range, which must of. course not be. absent. in. the:reproduction ofthe; whole acoustic spectrum, mustthen be reproduced by aseparate. high tone loudspeaker, in which. the distortion factor mustbeas low a's possible. A decreased outputinthe reproduc-- tion ofthis.range may betaken into-the bargain, since it is substantially notaudible.

Ithaspreviouslybeen suggested tosupply the low and high frequenciesseparately to two different output tubes, constituted-by a triode and apentode, respectively. However, this arrangement requires the use of a,comparaa tively'large-number of circuit elements.

According to the invention, inthe output stage of.-; a:. low-frequencyamplifier having: two loudspeakers preferablyintended" for thereproduction of lowfrequencies andhiglr. frequencies, respectively,of-the acoustic spectrum with the useof one. or more-screen-grid tubesas output tubes, the impedances; which are effected: by theloudspeakersinthe anode circuits of the; tubesare connected in series;The anode of each tubeis connected; to the. impedance: constitutedby'the: low-tone loudspeaker, which impedance is shuntedby acapacitorrof a' value such that theresistance of the capacitor for highfrequencies. is lowwith respect to this impedance. The screen-grid foralternating voltage is. connected. between the: two im pedances.-

I-n. order'that the-invention may bez-readil y' carried! into effect itwill now be. described, by way. ofexample, with reference; todheaccompanying drawing, wherein:

Fig. 1 is a schematic diagram of anembodiment of the circuit arrangementof the present: invention; and

Fig. 2 is a -schematic diagram of another" embodiment of the circuitarrangement" of the present invention.

In the circuit-arrangement of Fig; 1; the alternating voltage tobeamplified is; applied. to. control. grid 25- of pentode. 1. The:control: gridv receives: the.- requiredbiasing potentialiin the usualmanner by connecting: cathode; 3; via; a resistor 4 and. acapacitor. 5,,toground. potential; Suppressor grid.6. is connectedto. thecathode.The.-cir.- cuit:leading from. anode 7 to battery 8 is completedvia theprimary winding. 9. ofan output transformer 10,, the secondarywinding11015 which is.conne.cted to.-a.low-tone loudspeaker 12, and via theprimary winding 13,. con: nected'in. series with the winding 9, of anoutput' transformer 14, the secondary winding 15; of which is con- Theprimary winding 9 is; shunted by a capacitor 17" having a value suchthat it constitutes a very low impedance forihigh frequencies and hencesubstantially short-circuits output transformer for these frequencies.The screen grid 18 of pentode 1 isconnected directly to a point 19 ofthe anode circuit located between the two primary windings 9 and 13. Theprimary impedance 13 may be comparatively low, since an outputtransformer for a high-tone loudspeaker is concerned. For lowfrequencies, the screen grid 18 is thus substantially connected to theconstant positive potential determined by battery 8, so that tube 1operates as a screen-grid tube for low frequencies. Its output is verysatisfactory. However, the distortion, which is greater in screen gridtubes, is not completely heard in the loudspeaker, since the loudspeaker12 is a low-tone loudspeaker. For high frequencies, the transformer 10is substantially short-circuited by capacitor 17, which implies thatthe'anode 7 and the screen-grid 18 are substantially directly connectedto one another for these frequencies. The tube 1 thus operates as atriode for these frequencies and, while having a lower output for thesefrequencies, leads to a reproduction with a very low distortion factor.It is common knowledge in engineering to choose the proportioning suchthat the cooperation of the two loudspeakers yields the desiredsubstantially rectilinear reproduction curve of the whole arrangement.Of course, instead of utilizing the separate loudspeakers 12 and 16, usemay be made of groups of low-tone and high-tone loudspeakers eachcomprising a plurality of loudspeakers; it naturally being possible onlyto combine loudspeakers of the same kind into one group.

In the embodiment shown in Fig. l, the i-mpedances are constituted bythe transformers loaded with the loudspeakers. The situation remainsunchanged, if use is made of loudspeakers having impedances such thatthey can be included in the circuit without the use of transformers.Neither does a connection of the impedances free from direct currentchange anything in the fundamental operation.

Fig. 2 shows the use of the arrangement according to the invention inpush-pull connection. The two pushpull tubes 20 and 21 are shown onlydiagrammatically and the two halves 22 and 23 of the output transformer24 for the low-tone loudspeaker 25 are not connected directly to oneanother. Each half is bridged by a capacitor 26, 27, respectively, forhigh frequencies. The center tap 28 of the primary winding of outputtransformer 31 for the high-tone loudspeaker 32, which winding consistsof two halves 29 and 30, is connected to the positive terminal of thesource of anode supply. The anodes 34 and 35 of the tubes 20 and 21 areeach connected to one end of the primary windings 22 and 23, the otherends of these windings being connected to the primary winnings 29 and30. The screen-grid 36 of tube 20 is connected to point 37 between thewindings 22 and 29, the screen grid 38 of tube 21 being connected topoint 39 between the windings 23 and 30. The operation of the circuitarrangement of Fig. 2 is similar to that of the circuit shown in Fig. 1.

While the invention has been described by means of specific examples andin specific embodiments, I do not wish to be limited thereto for obviousmodifications will occur to those skilled in the art without departingfrom the spirit and scope-of the invention.

What is claimed is:

1. A low'frequency amplifier output circuit arrangement, comprising anelectron discharge device having an anode, a cathode and a plurality ofgrids including a screen grid, a first loudspeaker system forreproducing sound in a substantially low audio frequency range, a secondloudspeaker system for reproducing sound in a substantially high audiofrequency range, means for applying an input signal to one of saidgrids, means connecting said first and second loudspeaker systems inseries circuit arrangement and to said anode, said first loudspeakersystem being interposed between said anode and IT; p I 2,846,503

the impedance of said first loudspeaker system at said high audiofrequency range, means connecting said capacitor in shunt across saidfirst loudspeaker system, and means connecting said screen grid to acommon point between said first and second loudspeaker systems in saidseries circuit arrangement.

2. A low frequency amplifier output circuit arrangement, comprising anelectron discharge tube having an anode, a cathode and a plurality ofgrids including a screen grid, a first loudspeaker for reproducing soundin a substantially low audio frequency range, a second loudspeaker forreproducing sound in a substantially high audio frequency range, meansfor applying an input signal to one of said grids, first and secondcoupling transformerseach comprising primary and secondary windings,means connecting said first andsecond primary windings in series circuitarrangement and to said anode, said first primary winding beinginterposed between said anode and said second primary-winding, meanscoupling said first loudspeaker to said first secondary. winding, meanscoupling said second loudspeaker to said second secondary winding, acapacitor having a reactance value which is substantially. low withrespect to the impedance of said first primary winding at said highaudio frequency range, means connecting said capacitor in shunt acrosssaid first primary winding, and means connecting said screen grid to acommon point between said first and second primary windings in saidseries circuit arrange ment.

3. A low frequency amplifier output circuit arrange- I ment, comprisinga pair of discharge tubes connected in push-pull arrangement each havingan anode, a cathode and a plurality of grids including a screen grid, afirst loudspeaker system for reproducing sound in a substantially lowaudio-frequency range, a second loudspeaker system for reproducing soundin a substantially high audio frequency range, means for applying aninput signal to selected grids of said tubes, means connecting saidfirst and second loudspeaker systems in series circuit arrangementbetween the anodes of said tubes, said first loudspeaker system beinginterposed between said anodes and said second loudspeaker system, acapacitor having a reactance value which is substantially low withrespect to the impedance of said first loudspeaker system at said highaudio frequency range, means connecting said capacitor in shunt acrosssaid first loudspeaker system, and means connecting said screen grid toa common point between said first and second loudspeaker systems in saidseries circuit arrangement.

4. A low frequency amplifier output circuit arrangement, comprising apair of discharge tubes connected in push-pull arrangement each havingan anode, a'cathode and a plurality of grids including a screen grid, afirst loudspeaker for reproducing sound in a substantially low audiofrequency range, a second loudspeaker for reproducing sound in asubstantially high audio fre quency range, means for applying an inputsignal to selected grids of said tubes, a first coupling transformercomprising first and second primary windings and a secondary winding, asecond coupling transformer comprising primary and secondary windings,means connecting the primary windings of said first and secondtransformers in series circuit arrangement between the anodes of saidtubes, the primary winding of said second'transformer being interposedbetween the first and second primary windings of said first transformer,means coupling said first loudspeaker to said first secondary windingand means coupling said second loudspeaker to said second secondarywinding, a first capacitor having a reactance value which issubstantially low with respect to the impedance of the first primarywinding of said first transformer at said high audio frequency range, asecond capacitor having a reactance value which is substantially lowwith respect to the impedance of the second primary winding of saidfirst transformer at said high audio frequency range, means connectingsaid first capacitor in shunt across said first primary winding of saidfirst transformer, means connecting said second capacitor in shuntacross said second primary winding of said first transformer, meansconnecting the screen grid of said first tube to a common point betweensaid first primary winding of said first transformer and the primarywinding of said second transformer in said series circuit arrangement,and means connecting the screen grid of said second tube to a commonpoint between said second primary winding of said 10 5. A circuitarrangement as claimed in claim 4, further comprising a center tap onthe primary winding of said second transformer and means for applying asupply voltage to said tap.

References Cited in the file of this patent UNITED STATES PATENTS

